The present invention relates to adaptive power management in a modem. More particularly, the present invention relates to a method and apparatus for detecting and adapting to changing data rates in a telecommunications network using a modem. Such detection and adaptation can take advantage of reduced power modes in digital subscriber line communication networks, or other telecommunications schemes having reduced power modes.
Much interest has been expressed recently in DMT modems to increase bandwidth with various communication schemes, especially those digital subscriber line schemes commonly referred to as xDSL systems, such as ADSL. For example, asymmetric digital subscriber line (ADSL) was conceived originally for video-on-demand type applications, but the focus is now on providing higher speed Internet services, such as the World Wide Web. The asymmetry in ADSL refers to the allocation of available bandwidth and means that it is faster (i.e.xe2x80x94has more allocated bandwidth) in the downstream (towards the subscriber) direction and slower in the upstream (towards a central office) direction. Some applications, such as browsing on the Internet, do not generally demand symmetric data rates and can take advantage of an asymmetric system.
ADSL converts existing twisted-pair copper telephone lines into access paths for multimedia and high-speed data communications. ADSL can transmit more than 6 megabits per sec (Mbps) (optionally up to 8 Mbps) to a downstream subscriber from a central office, and as much as 640 kilobits per second (kbps) (optionally up to 1 Mbps) upstream from a subscriber to the central office. Such rates expand existing access modem capacities by a factor of 50, or more, without new cabling.
ADSL was designed for residential or small-office, home-office type services and thus, it was designed from the outset to operate with the analog voice signals of Plain Old Telephone Service (POTS) simultaneously on the same line, such that an additional copper line is not needed. Generally, the POTS channel is split off from the digital modem by filters to provide uninterrupted POTS, even if the ADSL circuit fails.
Unlike previous copper line technologies, an ADSL system does not need manual pre-adjustment to accommodate line conditions. Instead, the ADSL modem automatically analyzes the line, as part of the process of establishing a connection, and adapts itself to start up the connection. This adaptation process can continue, once the connection is started, as the modem compensates for ongoing changes, such as those due to temperature or other environmental factors. Factors that can affect ADSL transmission include the gauge thickness of the copper cable, the distance between the central office and the subscriber and the amount of interference present on the line.
To support bi-directional channels, ADSL modems can allocate the available bandwidth by FDM, where non-overlapping bands are assigned for the downstream and upstream data. DMT, which has now been accepted by ANSI as the standard line code for ADSL transmission, divides an input data stream among several sub-channels, each sub-channel having the same amount of bandwidth but at different center frequencies. Sub-channels can have different bit rates, as discussed below. Using many sub-channels with very narrow bandwidths means the theoretical channel capacity, as calculated according to Shannon""s law, can be approached. Generally, DMT was chosen because it is particularly well suited for transmission over copper line at the operating frequency bands. DMT also copes well with the typical noise and impulses that exist in the residential (subscriber) twisted-wire pair environment.
The sub-channels into which a channel is divided, commonly referred to as tones, are quadrature amplitude modulation (QAM) modulated on a separate carrier, commonly called a subcarrier, and the subcarrier frequencies are multiples of one basic frequency. The ANSI standard ADSL system has a theoretical maximum of 256 frequency sub-channels for the downstream data and 32 sub-channels for the upstream, though, in reality, line conditions, interference and other considerations reduce the actual available number of sub-channels. The frequency difference between two successive sub-channels is 4.3125 Khz. In a DSL-Lite or G.Lite system, the number of downstream data streams is halved, eliminating those at the higher frequencies.
As mentioned above, data to be transmitted is QAM modulated so that each sub-channel can transmit multiple bits and bit rates can vary between sub-channels. As the subscriber loops between the central office and a subscriber generally exhibit variations in gain and phase with frequency, each sub-channel can be arranged to carry a different number of bits appropriate for its frequency on the particular subscriber line. By assigning different numbers of bits to different sub-channels, each sub-channel can operate at an optimal, or near optimal, bit rate for the bandwidth available in the subscriber loop. Sub-channels at frequencies where the signal-to-noise ratio is low can have lower numbers of bits assigned to them, while sub-channels at frequencies with higher signal-to-noise ratios can have higher numbers of bits assigned to them, to keep the probability of a bit error constant across the subcarriers.
Generally, the actual user data traffic over a communication link established between two DMT modems is non-constant. The necessary bandwidth, data rate and event frequency can all vary. A data event is a single Protocol Data Unit (PDU) or a cluster of PDUs. In ATM, a PDU is a fixed length cell; in Internet Protocol, a PDU is a variable length IP packet A particular data event can be characterized as isochronous or asynchronous, and both data events can occur simultaneously over different channels, or groups of channels. A regular, or isochronous, data event, such as voice or compressed interactive video information, typically requires a relatively low bandwidth, but is not tolerant of delay greater than approximately 300 msec. xe2x80x9cBurstyxe2x80x9d, or asynchronous, data events, which are characteristic of interactive human-machine sessions such as world wide web sessions, can occur at random intervals, and can range from a low bandwidth and data rate requirement, such as a keystroke, to a high bandwidth and data rate requirement, such as a JPEG image transfer. In addition, very high bandwidth asynchronous data events, such as large file transfers and network backups, occur infrequently but require significant network resources, both in terms of data rate and bandwidth.
In the interest of conserving power and reducing system cooling requirements at the central office end, it is desirable to operate a DMT modem at a lower power when the data bandwidth is being underutilized. A number of power management states are defined in the current splitterless DMT ADSL (a.k.a. G.Lite or G.992.2) draft recommendation. In a xe2x80x9cfull onxe2x80x9d state (L0), the link is fully functional and the linked subscriber and central office modems are capable of delivering the maximum downstream and upstream rates possible under the given loop conditions, given the presence of any simultaneous active POTS devices and service provider restrictions. In the xe2x80x9cidlexe2x80x9d state (L3), the communication link is not active, and requires no power. Both the receiving and central office modems are transmitting idle (zero) signals. An optional xe2x80x9clow powerxe2x80x9d state (L1), is also defined, in which the communication link would be operational, but only require enough power to maintain the embedded operations control (EOC) channel and a low-rate data stream. State changes between full on, idle and low power management states are initiated under control of a higher layer function, typically at the application layer, and take on the order of hundreds of milliseconds to complete. As a result, these power management state changes are relatively infrequent when compared to the rate of change of actual user data traffic demands. In addition, since control of the power management state is dictated by higher layer functions it is not certain that the periods of low user data traffic can be exploited by the modem physical layer functions to save power.
For these reasons, an alternate power saving mode has been proposed for the next version of the DMT ADSL standard (G.992.2bis). This xe2x80x98quiescent modexe2x80x99 would offer similar power savings as the L1 and L3 power management states but would address the issues of rapid mode transitions and control at the modem physical layer. While entry into quiescent mode would require negotiation between the ATU-C and ATU-R (the CO and remote terminating units) taking on the order of 80 msec, a return to the high data rate L0 state (or potentially L1 (if that was the initial state)) would take only 1-2 DMT symbol periods (0.25-0.5 msec)). This would allow entry into and out of quiescent mode transparently of the higher layers (not impacting data throughput or delay). While in quiescent mode, there would be either no user data bandwidth on the link or a reduced user data bandwidth (transmitting in some (1 out of N) sub-set of symbol periods. There would be some overhead signal transmission to maintain link timing (e.g. pilot tone) and to monitor for channel variations.
The quiescent mode would be capable of exploiting gaps between user data traffic events where those gaps significantly exceed the time required to negotiate an entry into quiescent mode. Where the user data traffic is primarily due to isochronous data sources, such as interactive voice and/or video, the frame rate (e.g. typically 5-30 msec for voice) of the sources is such that there is not enough time between data events to negotiate entry into quiescent mode and as a result the modem must remain in the higher power L0 (or L1) state, even though the average user data rate may be much less than the modem link data rate. Buffering of the isochronous traffic into larger blocks of data with more time between blocks is not an acceptable option either, as this would require large buffers and, more importantly, would introduce significant delays (latencies) in the data connection which may not be tolerated in interactive voice communication.
The inability to exploit quiescent mode when the link is carrying only isochronous data traffic is a significant shortcoming for the likely cases where a user may have an interactive voice (e.g. voice-over-IP or voice-over-ATM) session running in parallel with a web-browsing session. Most of the time, there is only the regular, but relatively low-rate voice data traffic on the linkxe2x80x94only occasionally injected with a short, high throughput event such as a graphics-heavy web page download.
It is therefore desirable to provide a method and apparatus that permits a modem to operate in a reduced power mode when isochronous data is being transmitted over a communication link, and to detect and adjust quickly to new traffic conditions without recourse to higher application levels.
In a first aspect, the present invention provides a method for power management in a modem attached to a communications link. The modem, typically a discrete multi-tone modem, has a full on power mode and a quiescent power mode. The method consists of monitoring a communications link for incoming data traffic. If data traffic is detected on the link, it is tested to determine its periodicity, Typically, data arriving over a link is either asynchronous data or isochroous (quasi-periodic) data. The power mode of the modem is then determined based on the determined periodicity of the incoming data traffic.
The periodicity of the incoming data traffic can be determined either by performing a windowed autocorrelation, or a short-time Fourier transform on the incoming data traffic. Where a Fourier transform is used, the method can also include picking a peak and estimating a harmonic frequency of the incoming data traffic.
In a preferred embodiment, if the incoming data traffic is determined to be quasi-periodic, then the selected power mode is a quiescent power mode. Likewise, if the incoming data traffic is determined to be asynchronous, then the selected power mode is a full on power mode. Where the quiescent power mode is selected, the method can also include a further step of determining a minimum data rate at which to operate the modem.
To implement the method of the present invention, there is also provided a data traffic predictor and a dicrete multi-tone modem incorporating the traffic predictor. The traffic predictor comprises a data traffic monitor that detects incoming data traffic at the modem, a periodicity detector that determines if the incoming data is quasi-periodic, and a power mode controller for determining an appropriate power mode for the modem based on the determined periodicity of the incoming data traffic.
In accordance with an aspect of the present invention, there is provided a method for power management in a modem attached to a communications link. The modem includes a full on power mode and a quiescent power mode. The method includes the steps of: monitoring the communications link for incoming data traffic; analyzing a time series of the incoming data traffic and determining its periodicity; and selecting one of the power modes of the modem based on the determined periodicity of the incoming data traffic.
In accordance with a further aspect of the present invention, there is provided a data traffic predictor for a modem, for estimating data traffic over a communications link to permit power management in the modem. The modem includes a full on power mode and a quiescent power mode. The data traffic predictor includes; a data traffic monitor for detecting incoming data traffic at the modem and having means for determining a data arrival rate of the incoming data traffic; a periodicity detector for processing the data arrival rate and determining its periodicity; and a power mode controller for determining one of the power modes for operation of the modem based on the determined periodicity of the incoming data traffic.
In accordance with a further aspect of the present invention, there is provided a discrete multi-tone modem. The modem includes: a digital subscriber line transceiver having a full on power mode and a quiescent power mode; and a traffic data traffic predictor having a data traffic monitor for detecting incoming data traffic and having means for determining a data arrival rate of the incoming data traffic, a periodicity detector for processing the data arrival rate and determining its periodicity, and a power mode controller for determining one of the power modes for operation of the transceiver based on the determined periodicity of the incoming data traffic.